An automatic transmission which is mounted on a vehicle includes a plurality of frictional engagement elements such as clutches or brakes and realizes a plurality of speed gears by combining application and release of the frictional engagement elements. The automatic transmission includes a hydraulic circuit for supplying a working pressure individually to the frictional engagement elements. An hydraulic switch is provided in the hydraulic circuit for detecting a working pressure applied to the frictional engagement elements. Information of the hydraulic pressure detected by the hydraulic switch is used for feedback control of the working pressure.
In the automatic transmission, in the event that a further frictional engagement element is applied in addition to a preset application of a predetermined frictional engagement element, interlocking may be generated as a result of the simultaneous application. Therefore, a possibility of generation of the interlocking is determined based on the information on the hydraulic pressure detected by the hydraulic switch or the like. Further, a fail-safe operation in which the transmission is fixed to a predetermined speed gear is executed as a measure for avoiding the interlocking.
In the case where an electric system such as the hydraulic switch malfunctions, there may occur a case where the appropriate control of the working pressure that is applied to the frictional engagement elements becomes difficult and the possibility of generation of the interlocking is erroneously determined. A fail-safe operation to be executed as a measure for avoiding the erroneously determined interlocking is originally unnecessary. It is necessary to confirm whether or not the electric system such as the hydraulic switch operates normally in order to avoid as many unnecessary fail-safe operations like such an unnecessary one as possible.
A malfunction detection apparatus described in Patent Literature 1 detects a malfunction of a hydraulic switch in an automatic transmission quickly and clearly. Namely, the malfunction detection apparatus detects a state of the hydraulic switch when an ignition switch is on before an engine is started and determines that the hydraulic switch malfunctions in the event that the hydraulic switch continues to be on until a timer value TB reaches a predetermined value Tb. In this way, the state of the hydraulic switch is determined in such a state that no hydraulic pressure is surely generated before the start of the engine.